What is OMT (Orthomode Transducer) ?

The OMT is a device used in VSAT to satellite communication systems to separate or combine signals having different polarizations. As mentioned the full form of OMT is Orthomode Transducer. Satellite signals are often transmitted with specific polarization states (e.g. vertical and horizontal). OMT is designed to take two orthogonal polarized signals and split them into separate output ports, allowing each signal to be directed to appropriate transmitter or receiver. OMT enables simultaneous transmission and reception of signals with different polarizations using single antenna.

VSAT system composed of BUC, LNB, Satellite modem, Antenna and OMT. Let us understand transmit and receive process in VSAT system.
➨Satellite modulator(part of modem) converts baseband information to modulated IF frequency. This Modulated IF is upconverted and amplified using BUC (Block Up converter). The same BUC output is interfaced with antenna. BUC has functionality of both RF up converter and power amplifier.
➨The received RF signal from antenna is fed to LNB (Low Noise Block Converter). This LNB includes both LNB and Down converter functionality. Hence received signal is amplified and down converted. The down converted signal is demodulated using demodulator part of satellite modem.

OMT is a passive microwave device which allows interfacing of transmitter part as well as receiver part to be connected with the same feed horn. The same is shown in the following OMT diagram.

OMT-Orthomode Transducer

It is used in VSAT to satellite communication as well as satellite TV applications. It has connecting ports for two rectangular and one circular waveguide. In VSAT, OMT is used to handle uplink and downlink signals having different linear polarization orientations. The OMT device at VSAT terminal helps to separate these signals to ensure efficient communication.

Following technical specifications are considered for purchasing the OMT for VSAT and other applications.
• Waveguide Interfaces (Transmitter Port, Feedhorn Port, LNB Port)
• Frequency of operation in transmit and receive direction
• Return loss for transmit port and receive port
• Isolation between transmit and receive ports with same polarisation and orthogonal polarisation
• Insertion Loss between transmit port to receive port and between receive port to feedhorn port
• Maximum input power (Transmit port to feedhorn port)
• Operational temperature
• Weight

Application of OMT VSAT

As we know OMT is available for various frequency bands as per need such as C band, Ku band and Ka band. We will understand OMT used for C band VSAT application.

OMT VSAT application note

As shown in the figure-2, OMT is interfaced with two rectangular waveguide flanges and one circular flange. BUC with 6 GHz frequency band with horizontal polarisation is used in transmit side and LNB with 4 GHz frequency band with vertical polarisation is used in receive side. Feed horn is interfaced with circular port of the OMT(Orthomode Transducer). OMT acts as transmit reject filter when antenna is receiving the downlink signal.

OMT Manufacturers

• Smiths Microwave (www.millitech.com)
• Pasquali Microwave Systems (https://www.pasquali-microwavesystems.com/)
• Parabolic Networks (https://www.parabolicnetworks.com/)
• paradise
• Global Invacom Ltd. (https://www.globalprofessional.co.uk/)

Benefits of OMT

Following are the advantages or benefits of OMT.
➨It enables use of single antenna to handle different polarizations (e.g. horizontal and vertical) which reduces the need for multiple antennas. Hence it reduces the need for additional hardware and complexity of system.
➨It improves communication quality by preventing polarization cross-talk and reducing signal degradation.
➨OMT simplifies design and installation of satellite communication systems.
➨OMTs support signals in various bands which make them versatile components in VSAT to satellite communication systems.
➨OMTs contribute to space and cost savings.

Conclusion : Overall, the Orthomode Transducer plays a crucial role in maintaining signal integrity and minimizing interference in VSAT satellite communication systems by managing signals with different polarizations. OMTs are also used in radio astronomy, remote sensing and other microwave applications.

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